The present invention relates to the field of heater-drier stations for fixing toner images on sheet material.
The transportation of sheet material over a heated platen, having toner images to be fixed or fused thereon by the application of heat is well known in the art. It is old to pass such material over a curved blanket heater to produce sufficient heat by conduction upon the surface of the sheets to fix the images by fusion during the time interval of contact between the sheets and the curved platen. Many of these prior art curved heated platens have apertures therein wherein a vacuum is set up to suck the sheets against the heated surface of the platen to cause the sheets to remain in intimate contact with the heated platen during motion of the sheets through the fuser station. While these arrangements are often deemed satisfactory for fixing images on lighter, relatively flimsy electrostatic copy paper or plain paper, the use of a vacuum arrangement has been found to be undesirable where toner images are to be fixed by heat upon the surfaces of relatively heavy, stiff, wet photolithographic plates. This is because the vacuum required to cause the relatively heavy and stiff plates to adhere to the surface of the platen is considerable and is thus disadvantageous. Additionally, the large normal vacuum induced forces required to maintain the relatively stiff heavy plates in contact with the platen would in turn induce a high degree of sliding frictional resistance, particularly in the case of wet plates, which renders the driving of the plates through the heater station considerably more difficult.
It is thus an object of the present invention to provide a curved platen heater station for fixing toner images on relatively heavy, wet, stiff printing plates without the use of vacuum apertures formed in the heater platen.
In U.S. Pat. No. 3,517,164 to Huggins et al a curved heater-platen is disclosed together with a pair of input rollers which drives copy paper at the surface of the curved platen at an acute angle. The vacuum maintains the paper in contact with the surface of the platen and the leading edge of the paper passes through the bites of a pair of exit rollers so that the paper does not form an acute angle with respect to the surface portion of the platen adjacent the exit rollers. Since this reference employs vacuum to maintain contact of the paper with the platen it does not teach a practical solution for the above-stated problems solved by the present invention. In U.S. Pat. No. 4,059,394 to Ariyama et al, the leading edge of electrophotographic copy paper is directed at an acute angle with respect to the input surface of a heating platen. However, a toothed guide wheel is employed to force the leading edge of the paper downwardly to contact the major surface of the platen, and the leading edge thereafter passes over such surface through the bite of the exit roller pair. It is apparent, however, that the trailing edge portions of the sheets will not be firmly pressed against the platen (as required to fuse images on the relatively heavy plates) since the bite of the exit roller pair does not cause such trailing edge portions to form an acute angle with respect to the platen surface. Furthermore, this patent requires the use of a toothed wheel which must be rotated by a driving device to cause the leading edge to be forced downwardly toward the platen, and also requires a charge portion of platen surface portion 31a and is thus much more complex than the present invention. In contrast, the present invention employs the high resiliency of the relatively heavy and stiff plates to maintain the required intimate contact of all portions of the plates with the platen, without additional devices. Other less pertinent patents disclose various heater-platen arrangements such as U.S. Pat. No. 3,349,222; 4,075,456; 3,857,189 and 4,147,922.
Thus, it is an object of the present invention to provide a simple, reliable apparatus for fixing toner images by causing all portions of relatively heavy, stiff plates, including the leading and trailing edge portions thereof, to be in intimate contact with the surfaces of the heating platen. It is a further object of the present invention to eliminate transport guide members for controlling the path of motion of the printing plate over major platen surfaces, electrostatic charging techniques, or the use of vacuum techniques for maintaining sheet-platen contact.